Throughout the laboratory, mechanical testing of various types is conducted using pinned clevis fixtures to transmit test loads to a range of specimen configurations. The accuracy of such tests is a function of the test sample alignment relative to the axis of loading. Thus, to insure reliable and repeatable test results, fixture induced specimen bending must be kept to a minimum. ASTM provides specific recommendations as to measuring and minimizing misalignment for some specimen configurations. For example, a detailed description is given of an instrumented specimen and test procedure to establish less than 10% bending 30 KSI for sharp notched cylindrical tension specimens; for low cycle fatigue testing of axial configured specimens, maximum bending must be less than or equal to 5% of the minimum axial strain range.
However, ASTM is vague with respect to pin loaded specimens. For example, the ASTM alignment recommendation for fracture toughness testing of compact tension specimens is to minimize the eccentricity of the pull rods to within 30 mils. This is not always adequate to ensure the desired straight, uniform crack fronts. Misalignment should be maintained at less than or equal to 5%. Verification of this requirement has proven difficult.
One presently employed technique utilizes a modified compact tension specimen provided with a strain gage on either side of the preformed notch. Such a specimen is installed in the load train and individual strain readings are obtained over a given load range. The specimen is then reinstalled, following a 180.degree. rotation in the horizontal plane, allowing inherent specimen error due to machining, gage placement, etc. to be arithmetically canceled. Any remaining error in the various readings is then attributed to fixture misalignment.
In practice, this prior procedure has produced data which is inconsistent (non-repeatable) and difficult to interpret due to the many variables in a typical load train. In addition, the specimen configuration itself results in the following disadvantages: (1) machining tolerances in section thickness and hole placement or parallelism not tight enough for alignment standard requirements; (2) time consuming multiple orientation test requirements; (3) low strain outputs at typical test loads excessively weight small differences; and (4) inherent error due to typical relative gage placement errors.
In U.S. Pat. No. 2,290,868 (Eriksson), an apparatus for testing the tensile strength of materials is disclosed. Two equal test specimens are used with this apparatus, one of which is subjected to a tensile load while the other compensates for any temperature induced variations in length not related to the applied tensile load.
A method and apparatus for testing materials is disclosed in U.S. Pat. No. 2,356,763 (Keinath). The material to be tested is stressed in series with a standard piece according to the method. The behavior of the tested material is then related to the known behavior of the standard piece.
A tensile testing apparatus is disclosed in U.S. Pat. No. 2,534,980 (Lubahn). This apparatus utilizes an elongate test specimen which is divided into two different testing lengths provided between end cross heads and a middle head. The elongation under tension is measured between the two lengths located between the three heads, and the difference in the amount of the two elongations is equal to the elongation of the extra length provided on one side of the middle head.
In U.S. Pat. No. 2,617,293 (Schnadt), specimens for use in determining the brittleness of materials are disclosed. In one disclosed specimen, a cylindrical channel is drilled adjacent a test notch in the sample. A core member is then inserted in the channel to reduce the plastic deformation which would otherwise occur during testing of the specimen.
A fatigue testing apparatus for testing a least one pair of propellers is disclosed in U.S. Pat. No. 3,290,926 (Montana). The blades of the pair are mounted side by side and coupled together so that when a motor loads the pair of blades, the coupled blades are equally loaded.
A corrosion fatigue stressing apparatus and method is disclosed in U.S. Pat. No. 3,427,873 (Mehdizadeh). With this apparatus, a plurality of similar specimens are tested in a corrosive environment while the specimens are under stress.
An apparatus for fatigue testing a plurality of test rods is disclosed in U.S. Pat. No. 3,491,586 (Branger). The test rods are located about a pitch diameter and are simultaneously loaded in equal amounts with the apparatus.
In U.S. Pat. No. 3,680,367 (Krafft), a multispecimen fatigue cracking machine is provided for fatigue cracking a plurality of test specimens. The specimens are equiangularly located about a circular arbor so as to be repetitively flexed. A fatigue apparatus for fatigue testing a plurality of similar elements at respective test stations is also disclosed in U.S. Pat. No. 3,937,071 (Slota).
Other apparatuses and methods of general interest relating to tensile testing are disclosed in the following U.S. Pat. Nos.: 2,634,487 (Rogers); 3,228,238 (Jentet); 3,994,159 (George et al.); 3,918,299 (Donnadieu); 3,983,745 (Juusola); 4,003,246 (Cain); 4,090,489 (Barker); and 4,149,406 (Russenberger). Also of general interest is Soviet Union Pat. 800,799 and "Grip for Cyclic Testing of Compact Specimens Under Tension-Compression", E. A. Grin, Copyright 1981 by Plenum Pulishing Corporation No. 0019-8447/81/4706-0653.